Hydrogen bonds are formed between molecules containing a hydrogen atom bonded to a

Analyze the question: It asks what kind of atom hydrogen must be bonded to for hydrogen bonding to occur between molecules.

Concept: A hydrogen bond is a special, fairly strong type of dipole-dipole attraction. It forms when hydrogen is covalently bonded to a very electronegative atom, mainly fluorine (F), oxygen (O) or nitrogen (N). The reasoning goes like this:

1. The highly electronegative atom pulls the shared electrons towards itself.
2. Hydrogen is left with a strong partial positive charge (δ+), and the other atom has a partial negative charge (δ−).
3. The δ+ hydrogen is attracted to a lone pair on a strongly electronegative atom in a neighbouring molecule. This attraction is the hydrogen bond.

Examples: H₂O, NH₃ and HF.

Checking the options:

• A. Strongly electronegative atom. Correct. This is exactly the condition for hydrogen bonding.
• B. Non-polar species. Wrong. A non-polar bond (such as C–H in methane) has no significant δ+ on hydrogen, so no hydrogen bonding occurs.
• C. Diatomic element. Wrong. H₂, Cl₂ and similar molecules are non-polar and show only weak van der Waals forces.
• D. Complex ion. Wrong. This has nothing to do with the requirement for hydrogen bonding.

**Answer: A. Strongly electronegative atom.

Exam tip: Remember "FON": hydrogen bonding needs H attached to F, O or N. Hydrogen bonding explains why water has a high boiling point and why ethanol dissolves in water. Questions on these topics come up often.